This invention relates to modulators for use in a multi-carrier/time division multiple access system applicable to digital satellite communications and the like and more particularly to this type of modulator which can suppress the spread of a modulated wave spectrum during on/off switching of a carrier. In the multicarrier/time division multiple access system, a plurality of burst-like carriers are transmitted through a transmission line which is either a wire transmission line or a radio transmission line.
Conventionally, a modulator for use in the quadrature phase shift keying (QPSK) modulation/time division multiple access (TDMA) system has been known as shown in FIG. 1 on page 139 of Fourth International Conference on Digital Satellite Communications, Oct. 23-25, 1978, MONTREAL, CANADA, IEEE Catalog No. 78CH1326-8 entitled "A QPSK MODEM DESIGNED FOR 120 Mb/s TDMA TRANSMISSIONS WITH A 70 MHz INTERMEDIATE FREQUENCY". Also, a frame format of a burst-like signal is illustrated in FIG. 5 on page 102 of an article in the same literature entitled "AN APPROACH FOR THE DESIGN OF A TDMA BURST MODEM OPERATING IN A HIGHLY NONLINEAR SATELLITE CHANNEL".
The configuration of the known modulator and the known frame format of the burst-like modulated wave are illustrated herein in FIGS. 1 and 2, respectively. Referring now more particularly to FIG. 1, a frame formatting circuit 1 for the burst-like modulated wave responds to a binary I (channel) data signal 20, a binary Q (channel) data signal 22 and a timing signal 21 to produce a binary I bit pattern 23 and a binary Q bit pattern 24. The binary I bit pattern 23 and binary Q bit pattern 24 have each the form of a unipolar signal and are respectively converted by NRZ (Non Return to Zero) waveform conversion circuits 2 and 3 into bipolar NRZ waveform signals of .+-.1 polarities or senses. NRZ I bit stream 25 and NRZ Q bit stream 26 respectively delivered out of the NRZ waveform conversion circuits 2 and 3 are passed through band limit filters 4 and 5 so as to be converted into an I base band waveform signal 27 and a Q base band waveform signal 28 which are supplied to double balanced mixers 6 and 7. A carrier 36 generated from an oscillator 10 is applied to a phase shifter 12 which responds to the carrier 36 to provide carriers 29 and 30 of different phases. These carriers 29 and 30 are .pi./2 out of phase relative to each other and applied to the double balanced mixers 6 and 7, respectively. The mixers 6 and 7 then respectively deliver out an I binary phase shift keying (BPSK) modulated wave 31 and a Q BPSK modulated wave 32. The modulated waves 31 and 32 respectively correspond to the carriers 29 and 30 which are in their original phases when the output signals from the NRZ waveform conversion circuits 2 and 3 are of +1 polarity but they respectively correspond to the carriers 29 and 30 which are 180.degree. phase shifted relative to their original phases when the output signals from the NRZ waveform conversion circuits 2 and 3 are of -1 polarity. The orthogonal BPSK waves 31 and 32 are then added together at an adder 8 which in turn produces a QPSK modulated wave 33. In the above operation, the carrier 36 is on/off controlled using a carrier on/off signal 34 so that the QPSK wave 33 has the form of a burst-like modulated wave. The carrier on/off signal 34 may be obtained from the frame formatting circuit 1 or alternatively may be supplied externally. The carrier on/off signal 34 is delayed by a predetermined time interval .tau. at a delay circuit 9 so as to be converted into a delayed carrier on/off signal 35 which acts to on/off control a carrier on/off switch 11. The predetermined delay time .tau. is so determined as to permit the carrier on/off signal 34 to synchronize with the frame head and frame trail of the burst-like modulated wave, so that the delay circuit 9 can provide the carriers 29 and 30, which are switched on and off, with the same time lag as a time lag by which the I and Q binary data signals are delayed before they reach the double balanced mixers 6 and 7.
Referring now particularly to FIG. 2, the exemplary frame format of the burst-like modulated wave will be described. One frame has a plurality of burst-like modulated waves between which a guard time interval is provided. Each burst-like modulated wave is comprised of a preamble portion and a data portion and the preamble portion is comprised of a carrier recovery code (CR code), a timing recovery code (STR code) and a code word. To explain behavior of the burst-like modulated wave at its head with reference to FIG. 3, when the delayed carrier on/off signal 35 assumes on state at the CR head of the bit pattern 23 or 24 to switch on the carrier, the QPSK wave 33 as represented by its envelope is known to rise abruptly as illustrated in FIG. 3 at the instant that the carrier is switched on. Similarly, to explain behavior of the burst-like modulated wave at its trail with reference to FIG. 4, the QPSK wave 33 is known to fall abruptly at the instant that the carrier is switched off. The envelope components of the modulated wave change abruptly in this manner, causing a modulated wave spectrum to spread, as shown in FIG. 5, beyond its band having a center carrier frequency f.sub.c and a symbol time width Ts. More particularly, the abrupt envelope change leads to spreads of the spectrum as indicated by hatched areas which interfere with adjacent channels.
To determine the maximum (worst) amount of the interference due to the abrupt envelope change, a model as shown in FIG. 6 is considered. In this model, the burst-like modulated wave is assumed to be of a fixed code during an interval of N.Ts (N being constant integer). This fixed code has a spectrum amplitude A which is indicated in terms of the base band by the following expression: ##EQU1## indicating that the amplitude A changes with frequency f as graphically illustrated in FIG. 7.
Exemplarily, at f=1/Ts+1/(4NTs), the amount of the interference with adjacent channels is determined, for a sufficiently large N, from equation (1) as follows: ##EQU2##
Since the DC component of an NRZ pulse having a width Ts and a height of 1 (one) has an energy level of Ts.sup.2, the amount of the interference pursuant to equation (2) may be halved at the head and trail of the burst-like wave with the halved amount measuring a maximum of about -20 dB.
As described previously, in the conventional modulator shown in FIG. 1, the carrier rises and falls abruptly at the head and trail of the QPSK wave. Consequently, pulse-like noises appear in the adjacent channels due to the abrupt rise and fall of the carrier. One possible way to cope with this disadvantage may be such that the carrier level is controlled without changing the shape of the band limited spectrum. But this method would require that the envelope change unnecessarily extremely gradually over several symbols, inviting a decrease in transmission efficiency. A preferable countermeasure must satisfy the requirement that the envelope can change as abruptly as possible under less interference with the adjacent channels. It is to be noted that in the foregoing example the signal power level of the channel in question subject to the carrier on/off switching is assumed to be equal to that of the adjacent channels but obviously the higher the signal level of the channel in question relative to that of the adjacent channels, the more the amount of the interference increases.
FIG. 8 illustrates a known modulator which is intended to suppress the spread of the spectrum while permitting the abrupt rise and fall of the burst-like modulated wave. A modulator similar to the FIG. 8 modulator is disclosed in, for example, JP-B 63-4981 (published May 16, 1983).
The operation of the FIG. 8 modulator will now be described by making reference to waveforms illustrated in FIG. 9. In FIG. 8, reference characters a and b designate codes to be transmitted, c a first carrier on/off signal, d a second carrier on/off signal, 150 a local oscillator, 154 and 157 low-pass filters adapted to perform band limitation of the transmission codes, 151 a mixing circuitry, and 152, 153 and 156 switches. FIG. 9 illustrates at section (A) the transmission codes a and b each of which assumes "0" level or "1" level. Illustrated at section (B) in FIG. 9 is the first carrier on/off signal by which the switches 153 and 156 are turned off with its "0" level and turned on with its "1" level. FIG. 9 illustrates at (C) output waveforms of the low-pass filters 154 and 157, demonstrating that the on/off operation of the switches 153 and 156 causes the low-pass filters to undergo transient phenomena in the vicinity of on/off timing of the switches 153 and 156. FIG. 9 illustrates at (D) the output signal of the mixer 151, that is, a waveform (modulated waveform) of the carrier which is modulated with the output signals of the low-pass filters 154 and 157. Illustrated at (E) in FIG. 9 is the second carrier on/off signal by which the switch 152 is turned off or on with its "0" level or "1" level. FIG. 9 illustrates at (F) the output signal of the switch 152. In depicting the waveforms in FIG. 9, for clarity of explanation of the timing relation, time lags in the individual blocks are neglected. It is also noted that the transmission codes a and b actually take various bit patterns but these bit patterns are simply overlapped in the illustration at sections (A) and (C) in FIG. 9. The FIG. 8 modulator is disadvantageous in that two additional switches are needed as compared to the FIG. 1 modulator, that when the low-pass filters 154 and 157 are realized with digital circuits, the capacity of a ROM (Read Only Memory) used is increased imposing constraint on the hardware design, and that the guard time is lost by a time corresponding to one bit. To explain the second disadvantage specifically, the digital type low-pass filter is essentially comprised of a binary counter, a shift register, a ROM and a D/A converter and the transmission code (i.e. code to be transmitted) is first stored in the shift register. The ROM is precedently stored with sampled values of response waveforms by which the low-pass filter responds to various bit patterns of the transmission signal. The sampled values of the low-pass filter response waveforms corresponding to bit patterns stored in the shift register are then read out of the ROM sequentially as the value of the binary counter proceeds. The output signal from the ROM is converted by the D/A converter into an analog waveform. Generally, the digital low-pass filter is constructed in this manner. In applying this digital low-pass filter to the low-pass filter of FIG. 8, however, there is needed another shift register for storing the status of the first carrier on/off signal c and the ROM must be designed to store values corresponding to output signals of the additional shift register, leading to an increase in the capacity of the ROM. A low-pass filter structurally resembling the aforementioned digital type low-pass filter is disclosed in "C-2 ANALYSIS AND DESIGN OF A ROM SYNTHESIZER AS AN OPTIMUM DIGITAL TRANSMIT FILTER", pp 87-90, INTELSAT/TECE/TTE Third International Conference on DIGITAL SATELTTE COMMUNICATIONS, Nov. 11-13, 1975, KYOTO, JAPAN (See FIG. 16 of this application to be described later). To specifically explain the third disadvantage of the FIG. 8 modulator, since t described at (A) in FIG. 9 represents one-bit time width, the guard time is lost by t(=t/2+t/2) as is clear from (F) in FIG. 9.